The effects of cetane number, aromatics content and 90%
distillation temperature (T90) on HCCI combustion were investigated
using a fuel matrix designed by the Fuels for Advanced Combustion
Engines (FACE) Working Group of the Coordinating Research Council
(CRC). The experiments were conducted in a single-cylinder,
variable compression ratio, Cooperative Fuel Research (CFR) engine.
The fuels were atomized and partially vaporized in the intake
manifold. The engine was operated at a relative air/fuel ratio of
1.2, 60% exhaust gas recirculation (EGR) and 900 rpm. The
compression ratio was varied over the range of 9:1 to 15:1 to
optimize the combustion phasing for each fuel, keeping other
operating parameters constant.
The results show that cetane number and T90 distillation
temperature significantly affected the combustion phasing. Cetane
number was clearly found to have the strongest effect. An increase
in cetane number or a decrease in the T90 distillation temperature
advanced the combustion phasing. The cetane number effect is
related to increased low temperature heat release (LTHR) with
increasing cetane number. The T90 effect is primarily due to a
change in the physical delay period associated with preparation of
the fuel-air mixture. At a similar combustion phasing, the high CN
fuels exhibited significantly longer combustion duration than the
low CN fuels. The best fuel conversion efficiencies were generally
achieved with the four low cetane number fuels (FACE No. 1-4) as
optimized combustion phasing occurred at a higher compression
ratio. However, the two fuels with high CN and low T90 (FACE No. 5
and 7) also produced high fuel conversion efficiencies, although at
a slightly lower compression ratio. These two fuels (FACE No. 5 and
7) produced the lowest indicated specific HC (isHC) emissions. This
is due to the combined effects of high fuel volatility and
reactivity that resulted in a more efficient combustion process.
FACE No. 8, which has high CN and high T90, produced notably higher
isHC emissions when the combustion phasing was in the range of 0°
to 10°CA, aTDC. FACE No. 6 and 8, which have high CN and high T90,
produced higher isCO emissions when the combustion phasing was
retarded. The isNOx emissions were extremely low (below
0.008 g/kWh) for all fuels. Aromatic content did not directly
affect the combustion phasing or emissions behavior.